linux-kernel - Re: [PATCH 1/2] dt-bindings: usb: dwc3: Add binding for USB Gen2 de-emphasis

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Message-ID: <CAOSNQF0b--3o5bseU05Eu3a2zDiTTYfbnQNONFo3imw3HnaONA@mail.gmail.com>
Date: Tue, 8 Oct 2024 17:29:23 +0530
From: Joy Chakraborty <joychakr@...gle.com>
To: Krzysztof Kozlowski <krzk@...nel.org>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Rob Herring <robh@...nel.org>, 
	Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>, 
	Thinh Nguyen <Thinh.Nguyen@...opsys.com>, Felipe Balbi <balbi@...nel.org>, linux-usb@...r.kernel.org, 
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] dt-bindings: usb: dwc3: Add binding for USB Gen2 de-emphasis

On Tue, Oct 8, 2024 at 4:53 PM Krzysztof Kozlowski <krzk@...nel.org> wrote:
>
> On 08/10/2024 12:23, Joy Chakraborty wrote:
> > On Mon, Oct 7, 2024 at 8:26 PM Krzysztof Kozlowski <krzk@...nel.org> wrote:
> >>
> >> On 07/10/2024 15:55, Joy Chakraborty wrote:
> >>> PIPE4 spec defines an 18bit de-emphasis setting to be passed from
> >>> controller to the PHY.
> >>> TxDeemph[17:0] is split as [5:0] C-1, [11:6] C0, [17:12] C+1 for 3 tap
> >>> filter used for USB Gen2(10GT/s).
> >>>
> >>> Signed-off-by: Joy Chakraborty <joychakr@...gle.com>
> >>> ---
> >>>  Documentation/devicetree/bindings/usb/snps,dwc3.yaml | 12 ++++++++++++
> >>>  1 file changed, 12 insertions(+)
> >>>
> >>> diff --git a/Documentation/devicetree/bindings/usb/snps,dwc3.yaml b/Documentation/devicetree/bindings/usb/snps,dwc3.yaml
> >>> index 1cd0ca90127d..a1f1bbcf1467 100644
> >>> --- a/Documentation/devicetree/bindings/usb/snps,dwc3.yaml
> >>> +++ b/Documentation/devicetree/bindings/usb/snps,dwc3.yaml
> >>> @@ -190,6 +190,18 @@ properties:
> >>>        - 1 # -3.5dB de-emphasis
> >>>        - 2 # No de-emphasis
> >>>
> >>> +  snps,tx_gen2_de_emphasis_quirk:
> >>
> >> No underscores.
> >
> > Ack, will fix it with a follow up patch.
> >
> >>
> >>> +    description: When set core will set Tx de-emphasis for USB Gen2
> >>
> >> And why it cannot be implied by compatible?
> >
> > As per my understanding these are tuning coefficients for de-emphasis
> > particular to a platform and not the dwc3 controller, hence should not
> > be a controller compatible.
>
> Platforms must have specific compatible, so this should be implied by
> compatible.

Maybe I am using the word "platform" incorrectly here, what I
understand is that the same controller(in a chip) when used on 2
different physical form factors might need different deemphasis
coefficient values to be passed to its Phy. Someone could correct me
from the USB link stand point if I am mistaken here.

Thanks
Joy

>
> > Similar to the property defined right above this definition which is
> > from PIPE3 spec for USB Gen1.
>
>
> Best regards,
> Krzysztof
>